Membrane polymer compositions

ABSTRACT

The invention relates to a terpolymer of tetrafluoroethylene (TFE) monomer, polyvinylidene fluoride (PVDF) monomer and hexafluoropropylene (HFP) monomer for forming an ultrafiltration or microfiltration membrane, method of forming said membranes, and to the ultrafiltration or microfiltration membranes themselves. The invention also relates to a method of forming a polymeric ultrafiltration or microfiltration membrane including preparing a leachant resistant membrane dope which incorporates a leachable pore forming agent, casting a membrane from the dope and leaching the pore forming agent from the membrane. The invention also relates to a method of preparing a polymeric ultrafiltration or microfiltration membrane of improved structure including the step of adding a nucleating agent to the membrane dope before casting.

RELATED APPLICATION

[0001] This application is a continuation, under 35 U.S.C. § 120, ofInternational Patent Application No. PCT/AU02/00784, filed on Jun. 14,2002 under the Patent Cooperation Treaty (PCT), which was published bythe International Bureau in English on Dec. 27, 2002, which designatesthe U.S. and claims the benefit of Australian Provisional PatentApplication No. PR 5843, filed Jun. 20, 2001.

FIELD OF THE INVENTION

[0002] The invention relates to compositions suitable for use in formingmembranes, in particular for forming hollow fiber membranes for use inmicrofiltration. The invention also relates to membranes prepared fromsuch compositions, and to methods of their preparation.

BACKGROUND OF THE INVENTION

[0003] The following discussion is not to be construed as an admissionwith regard to the common general knowledge in Australia.

[0004] Synthetic membranes are used for a variety of applicationsincluding desalination, gas separation, filtration, and dialysis. Theproperties of the membranes vary depending on the morphology of themembrane i.e. properties such as symmetry, pore shape and pore size andthe polymeric material used to form the membrane.

[0005] Different membranes can be used for specific separationprocesses, including microfiltration, ultrafiltration, and reverseosmosis. Microfiltration and ultrafiltration are pressure drivenprocesses and are distinguished by the size of the particle or moleculethat the membrane is capable of retaining or passing. Microfiltrationcan remove very fine colloidal particles in the micrometer andsubmicrometer range. As a general rule, microfiltration can filterparticles down to 0.1 μm, whereas ultrafiltration can retain particlesas small as 0.01 μm and smaller. Reverse Osmosis operates on an evensmaller scale.

[0006] As the size of the particles to be separated decreases, the poresize of the membrane decreases and the pressure required to carry outthe separation increases.

[0007] A large surface area is needed when a large filtrate flow isrequired. One known technique to make filtration apparatus more compactis to form a membrane in the shape of a hollow porous fiber. Modules ofsuch fibers can be made with an extremely large surface area per unitvolume.

[0008] Microporous synthetic membranes are particularly suitable for usein hollow fibers and are produced by phase inversion. In this process,at least one polymer is dissolved in an appropriate solvent and asuitable viscosity of the solution is achieved. The polymer solution canbe cast as a film or hollow fiber, and then immersed in precipitationbath such as water. This causes separation of the homogeneous polymersolution into a solid polymer and liquid solvent phase. The precipitatedpolymer forms a porous structure containing a network of uniform pores.Production parameters that affect the membrane structure and propertiesinclude the polymer concentration, the precipitation media andtemperature and the amount of solvent and non-solvent in the polymersolution. These factors can be varied to produce microporous membraneswith a large range of pore sizes (from less than 0.1 to 20 μm), andaltering chemical, thermal and mechanical properties.

[0009] Microporous phase inversion membranes are particularly wellsuited to the application of removal of viruses and bacteria. Of alltypes of membranes, the hollow fiber contains the largest membrane areaper unit volume.

[0010] Flat sheet membranes are prepared by bringing a polymer solutionconsisting of at least one polymer and solvent into contact with acoagulation bath. The solvent diffuses outwards into the coagulationbath and the precipitating solution will diffuse into the cast film.After a given period of time, the exchange of the non-solvent andsolvent has proceeded such that the solution becomes thermodynamicallyunstable and demixing occurs. Finally, a flat sheet is obtained with anasymmetric or symmetric structure.

[0011] Hydrophobic surfaces are defined as “water hating” andhydrophilic surfaces as “water loving”. Many of the polymers that porousmembranes are made of are hydrophobic polymers. Water can be forcedthrough a hydrophobic membrane by use of sufficient pressure, but thepressure needed is very high (150-300 psi), and a membrane may bedamaged at such pressures and generally does not become wetted evenly.

[0012] Hydrophobic microporous membranes are characterized by theirexcellent chemical resistance, biocompatibility, low swelling and goodseparation performance. Thus, when used in water filtrationapplications, hydrophobic membranes need to be hydrophilized or “wetout” to allow water permeation. Some hydrophilic materials are notsuitable for microfiltration and ultrafiltration membranes that requiremechanical strength and thermal stability since water molecules can playthe role of plasticizers.

[0013] Currently, poly(tetrafluoroethylene) (PTFE), Polyethylene (PE),Polypropylene (PP) and poly(vinylidene fluoride) (PVDF) are the mostpopular and available hydrophobic membrane materials. Poly(vinylidenefluoride) (PVDF) is a semi-crystalline polymer containing a crystallinephase and an amorphous phase. The crystalline phase provides goodthermal stability whilst the amorphous phase adds some flexibility tothe membrane. PVDF exhibits a number of desirable characteristics formembrane applications, including thermal resistance, reasonable chemicalresistance (to a range of corrosive chemicals, including sodiumhypochlorite), and weather (UV) resistance.

[0014] While PVDF has to date proven to be the most desirable materialfrom a range of materials suitable for microporous membranes, the searchcontinues for membrane materials which will provide better chemicalstability and performance while retaining the desired physicalproperties required to allow the membranes to be formed and worked in anappropriate manner.

[0015] In particular, a membrane is required which has a superiorresistance (compared to PVDF) to more aggressive chemical species, inparticular, oxidizing agents such as sodium hypochlorite and toconditions of high pH i.e. resistance to caustic solutions.

SUMMARY OF THE INVENTION

[0016] According to a first aspect the invention provides the use ofpolymer suitable for forming into an ultrafiltration or microfiltrationmembrane, said polymer being a terpolymer of tetrafluoroethylene (TFE),PVDF and hexafluoropropylene monomers.

[0017] Preferably, the polymer includes from 20-65% PVDF monomer, from10-20% hexafluoropropylene monomer and 30-70% TFE.

[0018] More preferably, the polymer includes from 30-50% PVDF monomer,from 15-20% hexafluoropropylene, and from 30-55% TFE. Even morepreferably, the polymer includes from 35-40% PVDF and 17-20% HFP and40-48% TFE.

[0019] Most preferably, the polymer is a terpolymer of 44.6%tetrafluoroethylene (TFE) monomers, 36.5% PVDF monomers, and 18.9%hexafluoropropylene monomers.

[0020] Unless otherwise indicated, all percentages are expressed asweight percentages.

[0021] According to a second aspect the invention provides anultrafiltration and/or microfiltration membrane formed from a terpolymerincluding TFE monomers, PVDF monomer and hexafluoropropylene monomer.Preferably the monomer composition is approximately 44.6%tetrafluoroethylene (TFE) monomer, 36.5% PVDF monomer and 18.9%hexafluoropropylene monomer.

[0022] The membranes of the second aspect have an improved chemicalstability to oxidizing agents and caustic soda relative to a membraneformed from PVDF alone.

[0023] According to a third aspect the invention provides a method ofmanufacturing a microfiltration or ultrafiltration membrane includingthe step of casting a membrane from a composition including a terpolymerof 44.6% tetrafluoroethylene (TFE) monomer, 36.5% PVDF monomer and 18.9%hexafluoropropylene monomer.

[0024] Preferably, the membrane is in the form of a hollow fiber, castby the TIPS procedure, or more preferably by the DIPS procedure.

[0025] Most preferably, the polymer used is THV 220G, obtained fromDyneon® (3M) as a solvent soluble fluoropolymer. The polymer is acombination of approximately 44.6% tetrafluoroethylene (TFE) monomer,36.5% PVDF monomer, and 18.9% hexafluoropropylene monomer.

[0026] According to a fourth aspect, the invention provides a method offorming a polymeric ultrafiltration or microfiltration membraneincluding the steps of:

[0027] preparing a leachant resistant membrane dope;

[0028] incorporating a leachable pore forming agent into the dope;

[0029] casting a membrane; and

[0030] leaching said leachable pore forming agent from said membranewith said leachant.

[0031] Preferably, the leachant resistant membrane polymer includes aterpolymer of TFE, PVDF, and hexafluoropropylene. More preferably, thepolymer includes 44.6% tetrafluoroethylene (TFE) monomers, 36.5% PVDFmonomers, and 18.9% hexafluoropropylene monomers.

[0032] Preferably, the leachable pore forming agent is silica, and theleachant is a caustic solution, but the pore forming agent may forpreference be any inorganic solid with an average particle size lessthan 1 micron while the leachant may be any material/solution thatleaches the said pore forming agent from the membrane.

[0033] According to fifth aspect, the invention provides a method ofimproving the structure of a polymeric ultrafiltration ormicrofiltration membrane by the addition of a nucleating agent to amembrane dope. Preferably the nucleating agent is added in catalyticamounts and most preferably it is TiO₂, however, any insoluble/inert(unleachable) inorganic solid with an average particle size less than 1micron may be used.

[0034] According to a sixth aspect, the invention provides an elasticpolymeric ultrafiltration or microfiltration membrane having anasymmetric cross section defining a large-pore face and a small-poreface; said membrane having a higher flux at a given pressure from saidlarge-pore face to said small-pore face than from said small-pore faceto said large-pore face.

[0035] Preferably the elastic membrane is formed from the preferredmembrane forming mixtures of the preceding aspects, and may also beformed using the addition of leachable pore forming agents and/ornucleating agents.

[0036] The invention will now be described with particular reference tospecific examples. It will be appreciated, however, that the inventiveconcept disclosed therein is not limited to these specific examples

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] Membrane Formation

[0038] DIPS Procedure

[0039] THV 220G, obtained from Dyneon® Corp (3M) was dissolved inN-methylpyrrolidone (NMP) at approximately 20 wt. %. A flat sheetmembrane was cast from this solution and precipitated in water at 60° C.before being examined using scanning electron microscopy (SEM).

[0040] A standard DIPS process was employed as follows: Polymersolutions were mixed and heated to around 50° C. and pumped (spun)through a die into a 5 meter water-filled quench (or solidification)bath at 65° C. Non-solvent (lumen) consisting of 20% NMP, 10% water and70% polyethylene glycol (PEG200) was fed through the inside of the dieto form the lumen. The hollow fiber was then spun into the quench bathand solidified, before being run out of the bath over driven rollersonto a winder situated in a secondary water bath at room temperature tocomplete the quench and washing of the fiber.

[0041] The membrane structure was reasonable although a skin was foundon the surface of the membrane that prevented exposure of surface pores.

[0042] The caustic resistance of the membrane was tested by placing asample of the flat sheet into 5 wt. % caustic solution and comparing theappearance with a control of PVDF membrane cast by the TIPS process.

[0043] Both samples were thoroughly wet out with alcohol prior toimmersion in the caustic solution. The THV samples become transparentupon complete wetting. The results of the caustic immersion test areshown in table 1.

[0044] Table 1 shows the results of the caustic resistance tests. Theresults indicate that while the membranes are not impervious to caustic,as would be the case for a material like Teflon, they show extremelylimited degradation for an extended period of time in a comparativelystrong caustic solution. All subsequent exposures to 5% solutions haveshown the same result, that a slight yellowing occurs upon immediatecontact with the solution but no further degradation (either visually oraffecting the membrane properties) occurs.

[0045] In addition to color changes, the stiffness of both the PVDF andthe THV samples were examined. The PVDF membrane had lost a markedamount of flexibility and was quite brittle, while by contrast, the THVsample appeared to be relatively unaffected.

[0046] The results strongly suggest that no detrimental modification ofthe polymer membranes takes place as a result of such caustic immersion.TABLE 1 Date/Time elapsed THV 200 Sheet PVDF Fiber (TIPS)  5 minsColorless Light brown 10 mins Colorless Light brown  1 hr ColorlessDarker brown  2 days 19 hrs Colorless Dark brown/reddish  3 days Veryslight yellowing Very dark brown  3 days 18 hrs Very slight yellowingVery dark brown  4 days Very slight yellowing Very dark brown  5 daysVery slight yellowing Slightly darker/coppery  6 days Very slightyellowing Slightly darker/coppery  7 days Very slight yellowing Verydark turning black 10 days Very slight yellowing Very dark turning black11 days Very slight yellowing Very dark turning black

[0047] Modification of Membrane Hydrophobicity/Hydrophilicity

[0048] Those skilled in the art will appreciate the desirability ofpreparing membranes that are hydrophilic in character. For instance, asdescribed earlier hydrophilic membranes are simpler to operate thanhydrophobic membranes as they do not require an additional wetting step.

[0049] It was established in the present case that THV 220G iscompatible with Lutonal A25 (Polyvinylethylether) at concentrations ofaround 2%. Lutonal A25 makes the DIPS membranes of the presentapplication less hydrophobic.

[0050] Other than modifying hydrophobicity, the addition of Lutonal A25appeared to make little difference in the physical structure of themembrane, apart from opening the membrane structure slightly. Howevermembranes prepared with or without Lutonal are still acceptable in termsof their structure.

[0051] The addition of Lutonal A25 reduced the mixing time of the dopesquite dramatically.

[0052] Other elements of the DIPS process have also been investigated inconjunction with the use of THV 220G as a membrane polymer. It was foundthat non solvents can be used in a dope mix such as the addition of 5%glycerine triacetate (GTA) into the mixture without undue detrimentaleffects.

[0053] Leachable Dopants

[0054] In order to produce membranes without a dense surface skin andhaving a more hydrophilic nature, silica was added to the dope with theintention of leaching the silica out of the matrix by the use of acaustic solution.

[0055] A hydrophilic silica Aerosil 200 and a hydrophobic silica AerosilR972 were tested separately as additives to the THV 220G membranemixture. The dopes were cast into flat sheet membranes, and werequenched in hot water at 60° C. as described previously. Once themembranes had been cast, a portion thereof was leached in a 5% aqueouscaustic solution at room temperature for 14 hours. Without wishing to bebound by theory, it is believed that the silica reacts with caustic tomake the membrane hydrophilic as discussed below. Also, the leachingusing caustic soda provides a membrane of good open structure. A numberof membranes containing silica were cast. The results are shown in Table2. TABLE 2 Dope Hydrophilicity Dope Viscosity 18% THV, 8% Aerosil R972,Extremely hydrophilic Very high viscosity  2% Lutonal A25, 72% NMP 21%THV, 5% Aerosil 200, Hydrophilic Moderate (honey-like)  2% Lutonal A25,72% NMP viscosity 20% THV, 10% R972, Extremely hydrophilic Extremelyviscous (paste-  2% Lutonal A25, 68% NMP like) 20% THV, 5% R972,Extremely hydrophilic Moderate (honey-like) 75% NMP viscosity 20% THV,0.5% R972, Hydrophobic Low viscosity  2% Lutonal A25, 77.5% NMP 20% THV,80% NMP Extremely Hydrophobic Low viscosity 18% THV, 5% R972, Moderate(honey-like)  2% Lutonal A25, 75% NMP viscosity 20% THV, 5% R972, —Extreme viscosity - Far  5% Mg(OH)₂, 2% Lutonal too high to cast A25,68% NMP

[0056] Table 2 demonstrates that the silica is required in reasonablyhigh concentrations to make the membranes hydrophilic. It also shows thetrend of increasing viscosity with increasing silica content.

[0057] After the membranes were cast, and prior to leaching, themembranes were examined using scanning electron microscopy. Thestructures were generally extremely promising with the surface of thesheets completely open and totally free of any skin. The cross-sectionalappearance was more like a conglomerate of precipitated particles,rather than a true honeycomb like structure.

[0058] The best form of the silica appeared to be the hydrophobicAerosil R972, although both forms of silica produced a hydrophilicmembrane with a highly porous structure.

[0059] Subsequently placing the sample in caustic soda to leach thesilica provided a dramatic opening up in the membrane structure evenfurther. The result of the leaching was a change in the cross-sectionfrom the abovementioned conglomerate-like structure to the moretraditional lace or sponge-like formation.

[0060] The optimal dope for forming a DIPS polymer appears to be from amixture of 72% NMP, 20% THV, 6% silica and 2% Lutonal. This provides ahydrophilic membrane from a dope possessing a viscosity in the rangethat can be easily pumped.

[0061] A number of hollow fiber membranes were prepared from the abovedope. The wetting characteristics were as desired and the membranestructure showed an extremely open surface. While 6% silica Was used inthe present invention, it will be appreciated that the quantity can varysignificantly without departing from the present inventive concept.

[0062] Fibers incorporating silica with thicker walls were prepared andthe current properties of the fiber membranes were examined. The fiberwas then subject to leaching with a 5% caustic solution at roomtemperature for 18 hours.

[0063] It can be seen that leaching the membrane changes thepermeability and bubble points significantly without altering thedesirable physical properties of the membrane. The leaching of thesilica from the membranes has a positive effect upon permeability.

[0064] Thus, before leaching, the membrane had very few pores andextremely low flows. After leaching, however, the situation is reversedand there are a multitude of pores and a high flux.

[0065] A long leaching time is not necessarily required and can beincorporated in the production process as a post-treatment of the finalmodular product. The leaching process can be carried out at any time,however there is an advantage to postponing the leaching process as longas possible, since any damage to the surface of the fibers duringhandling can be overcome by leaching which physically increases theporosity of the membrane. Existing PVDF membrane surfaces can be damagedirreconcilably during production, resulting in a decrease inpermeability and flux of the fibers.

[0066] SEM analysis of the membranes showed a high degree of asymmetry.Asymmetry is defined as a gradual increase in pore size throughout themembrane cross-section, such that the pores at one surface of the hollowfiber are larger than the other. In this case, the pore size increasewas seen from the outer surface where the pores were smallest (and aquite dense surface layer was present) to the inner surface where thepores were significantly larger than those on the outer surface.

[0067] Preparation of the fibers was run at 65° C. rather than 50° C. asin a typical DIPS process. Increasing the quench bath temperature by10-15° C. dramatically affects the surface structure. The highertemperature gives a much more open surface. The use of the highertemperatures therefore accordingly means it is feasible to increase thepolymer concentrations and possibly the silica concentration if it isdesired to bolster the existing membrane and increase the mechanicalstrength.

[0068] Further it has been found that a more particular mixing procedurecontributes to the success of forming a membrane of high permeability.Mixing constituents together in a random manner does not produce such agood result as following a more stringent procedure whereby the AerosilR972 is dissolved in the total quantity of NMP and this solution isallowed to degas. The polymer pellets are mixed with the liquid LutonalA25 to coat the pellets. When these two procedures are complete, the twomixtures are combined. The advantage of this appears to be that thesilica is dispersed effectively and does not clump (which can lead tomacrovoids) and also, the pellets do not clump (which has the effect ofincreasing mixing time and consistency of the dope) since they arecoated with a sufficient quantity of Lutonal A25 for a sufficient timeto allow them to dissolve individually.

[0069] As well as silica, the leaching process allows for theintroduction of other functionalities into the membrane, such asintroducing hydrolyzable esters to produce groups for anchoringfunctional species to membranes.

[0070] Surprisingly, it has also been found that the membrane remainshydrophilic after leaching. Again, without wishing to be bound bytheory, the silica particles have a size in the order of manometers soconsequently the silica disperses homogeneously throughout the polymersolution. When the polymer is precipitated in the spinning process,there is a degree of encapsulation of the SiO₂ particles within thepolymer matrix. Some of the particles (or the conglomerates formed byseveral silica particles) are wholly encapsulated by the precipitatingpolymer, some are completely free of any adhesion to the polymer (i.e.they lie in the pores of the polymer matrix) and some of the particlesare partially encapsulated by the polymer so that a proportion of theparticle is exposed to the ‘pore’ or to fluid transfer.

[0071] When contacted with caustic, it is believed that these particleswill be destroyed from the accessible side, leaving that part of theparticle in touch with the polymer matrix remaining. The remainder ofthe silica particle adheres to the polymer matrix by hydrophobicinteraction and/or mechanical anchoring. The inside of the particle wallis hydrophilic because it consists of OH groups attached to silica.Because the silica is connected to hydrophobic groups on the other side,it cannot be further dissolved.

[0072] Thus, when the membranes are treated with caustic solution, thefree unencapsulated SiO₂ reacts to form soluble sodium silicates, whilethe semi-exposed particles undergo a partial reaction to form awater-loving surface (bearing in mind that given the opportunity, suchparticles would have dissolved fully). It is believed that the pores inthe polymer matrix formed during the phase inversion stage yet filledwith SiO₂ particles are cleaned out during leaching, giving a very open,hydrophilic membrane.

[0073] Nucleating Agents

[0074] TiO₂ (titania) was also added to the membrane at a variety ofconcentrations. TiO₂ has been added to membrane forming mixturespreviously as a filler to provide abrasion resistance or to act as anucleating agent, to increase the rate of fiber solidification.

[0075] However, surprisingly in the present case, it was found that theaddition of TiO₂ in concentrations below that used for reinforcement ofmembranes, a high degree of asymmetry was introduced into the membranes.In particular, this was as a result of the formation of a dense outerlayer. Without wishing to be bound by theory, the applicant believesthat the TiO₂ particles provide a site for phase inversion orprecipitation to begin. In hollow fiber membranes prepared by the DIPSprocess, the high number of fast solidification sites at whichprecipitation occurs means that the pores formed near the membranesurface are smaller, fewer and further between.

[0076] The use of too much titania can cause a dense outer layer on themembrane to restrict permeability. Further, as the titania dispersesvery well throughout the dope, only of the order of a catalytic amountis required. For example, only about 0.1-0.2 wt. % titania need beincorporated into the membrane, although as much as 3% can be useddepending on the desired effect.

[0077] A dope formulation giving good results is 20 wt. % THV 220G, 6wt. % Aerosil R972, 2 wt. % Lutonal A25, 0.2 wt. % TiO₂, and 71.8 wt. %N-methylpyrrolidone.

[0078] A dope having the above formulation was mixed and cast accordingto the DIPS method. They were then leached in 5% caustic soda solutionfor approximately 24 hours and then soaked in glycerol. Soaking fibersin glycerol or the like is a highly desirable step, since the materialis relatively flexible and will allow pores to collapse. The results forthe TiO₂ trial fibers are given as Table 3. TABLE 3 Property Results forTHV 200 Permeability (LMH) 3771 Bubble Point (kPa)  150 Burst Point(kPa) 150-160 Break Extension (%)  245 Break Force(N)   0.72 FiberDimensions (μM) 1080 OD, 535 ID Break Force per unit area (N/cm²)  105

[0079] Table 3 lists the properties of the membranes made whichincorporate a small proportion of TiO₂. The most apparent property tonote is the high permeability of the membrane.

[0080] High Polymer Concentrations

[0081] Attempts at making polymer concentrations above 20 wt. % wereattempted. Doing so however caused alternative problems mainly basedaround a dramatic increase in viscosity. Once the polymer portion risesto above 25 wt. %, viscosity becomes too high to pump in conventionalpumps. However, high polymer concentrations were seen to correlate withan increase in the mechanical strength of the membrane. Optimal resultsof workability and strength were achieved with the hollow fiber having apolymer concentration of 22%. The best was seen to be 22 wt. % THV 220G,6 wt. % Aerosil R972, 2 wt. % Lutonal A25 and 70 wt. %N-methylpyrrolidone. Concentrations as high as 30 wt. % polymer didproduce a feasible membrane. The high polymer concentration membraneswere leached in a 5% caustic solution for 24 hours and then soaked inglycerol. The results are shown in Table 4. A point of note is that theincrease in polymer concentration or the addition of TiO₂ does notappear to improve the bubble point or burst pressure of the fibers inany way. The mechanical strength of the fiber appears to be mainly afunction of wall thickness and lumen diameter. TABLE 4 Property Resultsfor THV 200 Permeability (LMH) 2821 Bubble Point (kPa)  150 Burst Point(kPa) 150-160 Break Extension (%)  240 Break Force (N)   0.64 FiberDimensions (μM) 930 OD, 542 ID Break Force per unit area (N/cm²)  145

[0082] Table 4 lists the properties of the membrane made using 22%polymer (without TiO₂). Comparing the results to Table 3, the membraneexhibit very similar characteristics with the exception that Table 3indicates possibly a higher permeability/flux for titania containingmembranes.

[0083] Physical Properties of Membranes

[0084] The bubble point measurements in Tables 3 and 4 do not give anentirely accurate determination of the bubble point, the pore size ormolecular weight cut off of the membrane because the membranes aresomewhat rubbery and flexible so that under pressure the membraneexpands and hence the pores stretch like a rubber band. It has beenobserved that the fibers increase in size slightly under a backwashpressure of as low as 100 Kilopascals.

[0085] This behavior is apparently due to the high elastic nature of thepolymer which also gives extremely high break tension described in Table4. This elastic behavior would adequately describe the apparently lowbubble point recorded for the membrane, since as the membrane isstretched by the pressure applied, the pores would be stretchingproportional to the overall size increase of the fiber. This property isextremely valuable for cleaning a membrane, since the pores may beopened up by the application of a liquid backwash and any materialfouling the pores may be easily dislodged and flushed away. The elasticbehavior also indicates that the membrane (and hence the pores) mayrecover up to 100% of such a deformation, thus the pores would return totheir original size.

[0086] To demonstrate this characteristic behavior, the permeability andfluxes of the fibers were measured. Permeability and flux are typicallymeasured with a filtration direction (direction of the filtrate flowrelative to the membrane surfaces) outside-in with the filtratecollected from the inside of the hollow fiber. To prove that the porestructure is increasing in size, the flow was reversed so that thefiltration direction was inside-out, with filtrate emerging on the outerside of the fiber.

[0087] Table 5 shows the results of these “outside-in” and “inside-out”tests TABLE 5 Pressure (kPa) Flux Outside-In (L/m² · hr) Flux Inside-Out(L/m² · hr) 22 919 1134 48 1550 2695 58 1374 3575 67 1327 4734 73 13535308 98 1322 7616 124 1283 11301

[0088] Table 5 and Figure 1 show that the flux for inside-out flowincreases as the pressure increases, while the outside-in flow remainsalmost completely constant. This indicates that the pressure appliedfrom the inside is expanding the pores to allow far higher flows. Thiselasticity described is one of the most desirable properties of themembranes discussed.

[0089] Potting

[0090] As a result of this one of the desirable features of themembranes according to the present invention is their ability to bepotted directly into epoxy. PVDF membranes require a more flexiblepotting material such as polyurethane to prevent damage to the fibers.PVDF fibers can break with relative ease if the fibers are potted in apotting material which lacks any flexibility. If there is no flexibilityin the potting material there can be breakage of the fiber at the pointwhere the fiber enters the pot. By contrast, the membranes of theinvention can be potted into epoxy potting material and the fibers willnot be significantly damaged during use. In fact, the membranes of thepresent invention can be stretched to the normal break extension of thefiber when pulled parallel to the pot surface i.e. 90° to the potteddirection.

[0091] The comparison of the properties of the THV membranes of thepresent application and PVDF prepared with the DIPS process are shown inTable 6. TABLE 6 THV 200 Property (after leaching) DIPS PVDFHydrophilicity Spontaneous Wetting Satisfactory Chlorine ResistanceHighly Resistant Resistant Caustic Resistance Highly resistant Noresistance Break Extension (%) 245 <145 Break Force (N)  0.73  <1.0Permeability (LMH @ 100 kPa) 3000-4000 ca. 300 Bubble Point (kPa) ca.150 ca. 350-400 Surface structure Extremely open Good AsymmetryExcellent Excellent

[0092] Table 6 gives a comparison between THV membranes manufacturedusing the DIPS process and the best (to date) PVDF membranesmanufactured using the DIPS process. The main differences are thespontaneous wetting of the THV membrane and also the high clean waterpermeability, both of which are lacking in current PVDF membranes. Theother difference lies in comparing the stiffness of the membranes, whichis directly attributable to the polymers used to produce the membrane.

[0093] It would be appreciated by those skilled in the art that whilethe invention has been described with particular reference to oneembodiment, many variations are possible without deviating from theinventive concept disclosed herein.

What is claimed is:
 1. A microfiltration or ultrafiltration membranecomprising a terpolymer derived from a tetrafluoroethylene monomer, apolyvinylidene fluoride monomer, and a hexafluoropropylene monomer. 2.The membrane according to claim 1, wherein the terpolymer comprises fromabout 35 wt. % to about 40 wt. % of repeating units derived from thepolyvinylidene fluoride monomer, from about 17 wt. % to about 20 wt. %of repeating units derived from the hexafluoropropylene monomer, andfrom about 40 wt. % to about 48 wt. % of repeating units derived fromthe tetrafluoroethylene monomer.
 3. The membrane according to claim 1,wherein the terpolymer comprises about 36.5 wt. % of repeating unitsderived from the polyvinylidene fluoride monomer, about 18.9 wt. % ofrepeating units derived from the hexafluoropropylene monomer, and about44.6 wt. % of repeating units derived from the tetrafluoroethylenemonomer.
 4. The membrane according to claim 1, the filtration membranehaving an asymmetric cross section defining a large-pore face and asmall-pore face, the membrane having a higher flux at a predeterminedpressure from the large-pore face to the small-pore face than from thesmall-pore face to the large-pore face.
 5. The membrane according toclaim 1, wherein the filtration membrane comprises a hollow fiber. 6.The membrane according to claim 1, further comprising a hydrophobicitymodifying agent.
 7. The membrane according to claim 6, wherein thehydrophobicity modifying agent comprises a polyvinylethylether.
 8. Themembrane according to claim 6, wherein the membrane comprises about 2wt. % of the hydrophobicity modifying agent.
 9. A method of casting apolymeric membrane, the method comprising: preparing a membrane dope,wherein the membrane dope is resistant to a leachant, and wherein themembrane dope comprises a leachable pore forming agent; casting afiltration membrane from the membrane dope; and leaching the leachablepore forming agent from the membrane with the leachant.
 10. The methodaccording to claim 9, wherein the membrane dope comprises a terpolymerderived from tetrafluoroethylene monomer, polyvinylidene fluoridemonomer, and hexafluoropropylene monomer.
 11. The method according toclaim 10, wherein the terpolymer comprises from about 35 wt. % to about40 wt. % of repeating units derived from the polyvinylidene fluoridemonomer, from about 17 wt. % to about 20 wt. % of repeating unitsderived from the hexafluoropropylene monomer, and from about 40 wt. % toabout 48 wt. % of repeating units derived from the tetrafluoroethylenemonomer.
 12. The method according to claim 10, wherein the terpolymercomprises about 36.5 wt. % of repeating units derived from thepolyvinylidene fluoride monomer, about 18.9 wt. % of repeating unitsderived from the hexafluoropropylene monomer, and about 44.6 wt. % ofrepeating units derived from the tetrafluoroethylene monomer.
 13. Themethod according to claim 9, further comprising the step of adding apolyvinylethylether to the membrane dope as a hydrophobicity modifyingagent.
 14. The method according to claim 9, wherein the leachable poreforming agent comprises an inorganic solid having an average particlesize of less than about 1 micron.
 15. The method of claim 9, wherein theleachable pore forming agent comprises leachable silica, and wherein theleachant comprises a caustic solution.
 16. A method of casting apolymeric membrane, the method comprising: preparing a membrane dopecomprising a polymer and a catalytic amount of a nucleating agent; andcasting a filtration membrane from the membrane dope.
 17. The methodaccording to claim 16, wherein the nucleating agent comprises aninsoluble inorganic solid or an inert inorganic solid.
 18. A methodaccording to claim 16, wherein the nucleating agent has an averageparticle size of less than about 1 micron.
 19. A method according toclaim 16, wherein the nucleating agent comprises TiO₂.
 20. The methodaccording to claim 16, wherein the polymer comprises from about 35 wt. %to about 40 wt. % of repeating units derived from the polyvinylidenefluoride monomer, from about 17 wt. % to about 20 wt. % of repeatingunits derived from the hexafluoropropylene monomer, and from about 40wt. % to about 48 wt. % of repeating units derived from thetetrafluoroethylene monomer.